1. Field of the Invention
The present invention relates to portable wireless communication systems, and more particularly to portable wireless communication systems suitable for use in restricted area, e.g., indoor use in a building or the like.
2. Description of The Prior Art
To further enhance mobile telephone systems for the purpose of obtaining a broader service area of wireless telephones, so-called indoor portable wireless communication systems have been widely studied which systems allow wireless telephones to be used indoors at buildings, underground streets, airports or the like.
Different from the mobile telephone system, the characteristics of indoor portable wireless communication is available at any location within a total service area extending indoors at a building or on the premises of a factory, and that such communication is allowed to be conducted while a subscriber station is moving.
With the progress of practical use of such wireless communication systems, it may well be expected that all indoor extensions are made cordless. Thus, terminal density of indoor portable wireless communication systems may become extensively higher than that of current mobile telephone systems.
In view of the above-described background art, a technique of using the allocated radio frequency resources effectively and to the extremity has been desired. Therefore, it becomes necessary to use a small service area so that reuse efficiency of a same frequency can be improved.
Technique required for such portable wireless communication systems are basically almost analogous to those required for conventional mobile telephone systems. However, the following channel allocation and hand-off techniques are different.
One of the physical differences of indoor portable wireless communication systems from conventional mobile telephone systems is the total number of base stations. As discussed above, it becomes necessary for indoor portable wireless communication systems to install base stations at very close intervals, resulting in a very large total number of base stations.
Therefore, it is difficult for a frequency management controller installed at the center of systems to control by itself the frequency management for all the base stations.
In addition, the indoor radio propagation characteristics are often unknown. Even if they are known previously, it may be expected that they change to large extent due to a change of an indoor layout or the like. It is difficult accordingly to provide a fixed cell structure as seen in conventional mobile telephone systems, where base stations are installed systematically.
To solve the above problems, a portable wireless communication system has been proposed as in Japanese Patent Laid-open application No. JP-A-61-244137, which system performs "automatically" the management and allocation of radio frequencies or channels in such a manner that each of the base stations disposed at irregular intervals independently judges what channels are available, and determines a priority order in which the channels should be selected when used for communication with a subscriber station based on learning from the past history.
The proposed portable wireless communication system, however, performs an autonomic processing only for the operation of channel allocation. Other technical factors required for the processings before the channel allocation, such as the call initiation from a subscriber station for determining the base station which is to be communicated with the subscriber station, the hand-off of the communication exchange according to movement of a subscriber station and the like, cannot be autonomically processed without a help of the control by an additionally installed management controller.